The invention relates to a system and method for controlling the air/fuel ratios of cylinders in an internal combustion engine.
To improve engine thermal efficiency and increase vehicle fuel economy, a lean burn engine approach has been used. In these system, the engine is operated lean of stoichiometry and relatively unthrottled, thereby decreasing engine pumping work and further increasing fuel efficiency. To increase the range of lean operation, direct injection is used where fuel is injected directly into the engine. In particular, when operating in the stratified mode, which is always lean of stoichiometry, fuel is injected during a compression stroke. Alternatively, when operating in the homogeneous mode, which can be either rich or lean of stoichiometry, fuel is injected during an intake stroke.
Since the engine operates lean for extending periods, and is therefore outside the three way catalyst operating window, a lean NOx trap, or adsorbent, is used. The NOx trap adsorbs NOx when operating lean, and release NOx when operating at or rich of stoichiometry. The NOx trap can also adsorb sulfur, which reduces NOx storage capacity and therefor reduces system efficiency.
One method for removing sulfur requires raising the NOx trap temperature to elevated levels and operating the air/fuel ratio at or rich of stiochiometry. Various methods are known for elevating the NOx trap temperature. One particular method operates some cylinders lean and some cylinders rich. When these two exhaust streams meet at the NOx trap, an exotherm is created thereby heating the NOx trap. In this way, NOx trap temperature is elevated and sulfur can be removed thereby replenishing the NOx storage capacity. Such a system is described in U.S. Pat. No. 5,758,493.
The inventors herein have recognized a disadvantage with the above approach. For example, when some cylinders are operated rich and some are operated lean, there is a torque imbalance since all of the cylinders receive equivalent airflow. This torque imbalance, or fluctuation, causes increased vibration and degrades customer satisfaction. One method to reduce the torque imbalance uses retarded ignition timing in the rich cylinders. The inventors herein have recognized that when using ignition timing to solve the torque imbalance disadvantage, a degradation in fuel economy is experienced since ignition timing retard is used. In other words, fuel economy is reduced since the cylinders operating rich do not produce the full potential torque that could be produced if optimum ignition timing were employed.
An object of the invention claimed herein is to provide a method for operating some cylinders lean and some cylinders rich while suppressing engine torque fluctuations and maximizing fuel economy.
The above object is achieved and disadvantages of prior approaches overcome by a method for controlling and engine, the engine having a first group of cylinders and a second group of cylinders, each coupled to an intake manifold, the method comprising: providing a base air amount and a base fuel amount in stoichiometric proportion to the first group of cylinders and to the second group of cylinders; adding excess fuel in addition to said base fuel amount to the first group of cylinders thereby providing a rich air/fuel ratio in the first group of cylinders; and adding excess air in addition to said base air amount to the second group of cylinders thereby providing a lean air/fuel ratio in the second group of cylinders.
By adding excess air to the lean cylinder, engine torque is mostly unaffected since the additional air does cause any extra fuel to be burned in the cylinder. Similarly, by adding excess fuel to the rich cylinder, engine torque is mostly unaffected since the additional fuel does not burn in the cylinder. In this way, it is possibly to maintain a balanced engine torque between lean and rich cylinders while providing lean and rich gases to an engine exhaust.
An advantage of the above aspect of the invention is improved fuel economy while operating with differing air/fuel ratios.
Another advantage of the above aspect of the present invention is improved engine smoothness.
In another aspect of the present invention, the method comprises a method for controlling and engine, the engine having a first group of cylinders and a second group of cylinders, the engine coupled to an intake manifold, the engine further having an outlet control device for controlling flow exiting the manifold and entering the first group of cylinders, comprising: providing a base air amount and a base fuel amount in stoichiometric proportion to the first group of cylinders and to the second group of cylinders; adding excess fuel in addition to said base fuel amount to the first group of cylinders thereby providing a rich air/fuel ratio in the first group of cylinders; and adding excess air in addition to said base air amount to the second group of cylinders by adjusting the outlet control device thereby providing a lean air/fuel ratio in the second group of cylinders.
By using the outlet control device, it is possible to have some cylinders operating at a different air/fuel ratio than other cylinders while having each cylinder producing equal torque. In other words, in the example when some cylinders operate lean and some cylinders operate rich, extra air is only added to the lean cylinders and extra fuel is added only to the rich cylinders.
An advantage of the above aspect of the present invention is improved engine smoothness.
Another advantage of the above aspect of the present invention is improved customer satisfaction.
In another aspect of the present invention, the above object is achieved and disadvantages of prior approaches overcome by a method for controlling and engine, the engine having a first group of cylinders and a second group of cylinders, the engine coupled to an intake manifold, the engine further having an outlet control device for controlling flow exiting the manifold and entering the first group of cylinders, the method comprising: requesting a split air/fuel operating mode; in response to said request, operating the first group of cylinders at a first air/fuel ratio; operating the second group of cylinders at a second air/fuel ratio; and adjusting the outlet control device to minimize a torque imbalance between the first group of cylinders and the second group of cylinders.
By adjusting the outlet control device to balance engine torque when operating in a split air/fuel mode, it is possible to provide exhaust gases with differing air/fuel ratios.
An advantage of the above aspect of the present invention is improved engine smoothness.
Another advantage of the above aspect of the present invention is improved customer satisfaction.
Yet another advantage of the above aspect of the present invention is improved emissions.
In still another aspect of the present invention, the above object is achieved and disadvantages of prior approaches overcome by a method for controlling temperature of an emission control device coupled to an engine, the engine having a first group of cylinders and a second group of cylinders, the engine coupled to an intake manifold, the engine further having an outlet control device for controlling flow exiting the manifold and entering the first group of cylinders, the method comprising: determining a first air/fuel ratio based on the temperature; determining a second air/fuel ratio; operating the first group of cylinders at said first air/fuel ratio; operating the second group of cylinders at said second air/fuel ratio; and adjusting the outlet control device to minimize a torque imbalance between the first group of cylinders and the second group of cylinders.
By providing exhaust gases with differing air/fuel ratios to the emission control device, exothermic reactions can control temperature with balanced engine torque.
An advantage of the above aspect of the present invention is improved engine smoothness.
Another advantage of the above aspect of the present invention is improved customer satisfaction.
In still another aspect of the present invention, the above object is achieved and disadvantages of prior approaches overcome by a method for decontaminating an emission control device coupled to an engine, the engine having a first group of cylinders and a second group of cylinders, the engine coupled to an intake manifold, the engine further having an outlet control device for controlling flow exiting the manifold and entering the first group of cylinders, the method comprising: indicating contamination of the emission control device; in response to said indication, operating the first group of cylinders at a first air/fuel ratio based on the temperature; operating the second group of cylinders at a second air/fuel ratio based on the temperature; and adjusting the outlet control device to minimize a torque imbalance between the first group of cylinders and the second group of cylinders.
By providing exhaust gases with differing air/fuel ratios to the emission control device, exothermic reactions can increase temperature and decontaminate the emission control device while also providing balanced engine torque.
An advantage of the above aspect of the present invention is improved engine smoothness.
Another advantage of the above aspect of the present invention is improved emissions.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.